Divide the experience up between your characters and it probably comes out right (with some of the adjustments that come with various skills and the like)
Do you have the Enchant Skill?? You need to have that as well as have the spells learned as well.
The Artifacts skill is for identification, not enchanting.
Oh sigh - yes, you can kill the dragon and whole bunch of other NPCs except the one that deserves it (Sebastio) but this is only possible when they have no other part to play in the rest of the game. You're having fun, aren't you??
Wyv